Iodine Biogeochemical Cycle and Microbial Bioremediation of Radioactive Iodine-129

Iodine is an essential biophilic element that plays pivotal roles in both environmental systems and human physiology, particularly as a key constituent of thyroid hormones and a regulator of atmospheric ozone. In contrast, its radioactive isotope, iodine-129 (I-129), predominantly generated through anthropogenic nuclear activities, represents a persistent environmental and public health concern. With an exceptionally long half-life of approximately 15.7 million years and high environmental mobility, especially in groundwater, combined with a strong tendency to bioaccumulate in the human thyroid, I-129 poses a disproportionate and long-term radiological hazard in contaminated sites. The biogeochemical cycling of iodine involves intricate interconversions among multiple oxidation states and phases across the lithosphere, hydrosphere, atmosphere, and biosphere. Microorganisms are central to these processes, mediating oxidation, reduction, methylation, accumulation, and sorption. While microbial methylation can increase I-129 mobility via the production of volatile methyl iodide, other microbial pathways, notably biosorption and binding to organic matter, provide promising mechanisms for immobilization and natural attenuation. Microbial bioremediation offers a sustainable and cost-effective alternative or complement to conventional physicochemical methods for managing radioactive contaminants. Strategies such as bioreduction, biosorption, bioaccumulation, and biomineralization exploit the metabolic versatility of microorganisms to alter radionuclide speciation, solubility, and mobility. However, practical application to I-129 remains challenging due to its extreme persistence, environmental variability, and uncertainties in predicting its long-term geochemical fate. Effective management of I-129 contamination will require an integrated, multidisciplinary approach that combines advanced microbial ecology insights, optimized biotechnological processes, and long-term monitoring frameworks.